Thermally stable cellulose products



United States Patent 3,510,346 THERMALLY STABLE CELLULOSE PRODUCTS Anthony J. Palumbo, Sharon, and James G. Ford, Sharpsville, Pa., assignors to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Filed Oct. 21, 1966, Ser. No. 588,304 Int. Cl. D06m 13/34; D21d 3/00; D21k N US. Cl. 117154 4 Claims ABSTRACT OF THE DISCLOSURE A thermally stable and durable cellulosic insulation material for use in a transformer in the presence of air and a liquid dielectric, the material may be cellulosic paper, having from about 0.02% to about 10% by weight of at least one nitrogen-containing stabilizer consisting of an amino and cyano substituted organosilane which stabilizer may exist in a monomeric, polymeric, or mixed form.

This invention relates to electrical apparatus embodying cellulosic insulation and, in particular, to the stabilization of such insulation in contact with liquid dielectrics.

At temperatures in excess of 100 C. cellulosic materials deteriorate relatively rapidly when in contact with air. Deterioration is accelerated at such temperatures when cellulosic materials are in contact with liquid dielectrics such as transformer oil, especially when the oil is permitted to be oxidized. For this reason, electrical apparatus employing cellulosic insulation generally is not operated continually at temperatures above 105 C.

The ability of the cellulosic insulation to retain appreciable mechanical and electrical strength while being operated at elevated temperatures is one factor involved in determining the temperature at which the apparatus may be operated safely. Such temperature limitation applies to cellulosic insulation in contact with or immersed in liquid dielectric though it is a problem when cellulosic insulation is heated in contact with air.

Patent No. 2,722,561 discloses that substances like urea and non-acidic compounds derived from urea, when added to oil filled transformers, greatly improve the thermal life of the cellulosic insulation contained therein. The patent broadly discloses that the addition of about 3% of urea (or one of its equivalents) to the oil, based on the total weight of the oil, provides paper having about the same life at 125 C. as it has at 100" C. without the urea stabilizer being present. The effective life of the cellulosic material is measured in terms of retention of tensile strength and/or retention of bursting strength.

There are reasons for incorporating the stabilizing compound directly in the cellulosic material rather than in the oil. Among other things, better control of the stabilizer and improved effectiveness is obtained when it is incorporated in the fibrous structure or body of cellulosic material.

In accordance with this invention, it has been found that certain nitrogen-containing organic compounds such as amines and nitriles can be incorporated in cellulosic materials to enhance their resistance to deterioration and enable their use at higher temperatures.

Accordingly, it is a general object of this invention to provide a thermally stable cellulosic product for use as insulation in contact with liquid dielectrics by incorporating in the cellulosic material certain nitrogen-containing stabilizing compounds.

It is another object of this invention to satisfy the foregoing problems and desiderata in a simple and effective manner.

Briefly, the present invention comprises a thermally 3,510,346 Patented May 5, 1970 ice R4 and where X is a nitrogen-containing amino (NH or cyano (CN) group. In the R structure above, the amino group would be attached directly to the silicon atom when n=0. R represents an alkyl or aryl radical. R R and R, can represent a monovalent alkyl, alkoxy, aryl or aryloxy radical. n can be 0 or 1 and m is from 0 to 25.

These nitrogen-containing compounds can exist in monomeric form as in (a) above. They can be used in the polymeric form (b) by substituting an OH group for any one R R or R followed by condensation through the OH groups to produce the (b) polymer. The preferred amount of stabilizer (a) or (b) or mixtures of both is from 0.5 to 5% of the weight of the cellulose material. The stabilizers are preferably distributed throughout the interstices within the cellulosic paper by being incorporated in the paper fibers during the process of making the paper. However, the stabilizers can be added to the paper, cloth, or pressboard by soaking them in a solution of the stabilizer, and drying to remove the solvent which latter may be alcohol, water or other volatile liquid.

In accordance with the present invention, it is now possible to reduce the rate of degradation and to increase the retention of both dielectric strength and mechanical strength of cellulosic insulation at elevated temperatures by substantially uniformly distributing throughout the insulation effective amounts of the above nitrogen-containing chemical stabilizing compound (a) or (b). Although the amount of a compound present may be small, it nevertheless imparts a highly beneficial stabilizing effect to the electrical insulation.

As an example, one of the nitrogen-containing compounds which has been found to impart these benefits effectively is gamma-amino propyltriethoxy silane wherein X in the above formula is the amino radical, NH and having the following formula:

cm-Nom-s iO I I (E2115 and NHzCHzOH2CHzSiO- i-CzH 2H5 CzHs The specific structure of the alkyl group can be varied and the radical substituted such as NH on any one of the carbon atoms of the R group to give an alpha, beta, gamma, delta, etc., amino substituted compound. Moreover, an amino group can be substituted on other alkyl or aryl groups, if present, and the amino group can be a primary or secondary type, or combination thereof.

Generally, these compounds exhibit substantial solubility in water or water and alcohol solutions which considerably enhances their incorporation into cellulosic insulation especially from the economic standpoint.

The benefits of the additives of this invention are dependent upon several factors. First, the stabilization compounds must be present in the cellulosic insulation in amounts within the range of about 0.02% to about by weight based on the weight of the cellulosic material. Less than 0.02% stabilization compounds does not impart to the insulation any appreciable improvement in either electrical insulation or mechanical strength at elevated temperatures. The presence of more than about 10% of the compounds is both difiicult to produce and uneconomical, because it does not appreciably increase the degree of improvement beyond that obtained with 10%. Within this broad critical range, it is preferred to incorporate in the cellulose about 0.5% to about 5% of the stabilization compounds, inasmuch as these amounts have been found to impart the optimum desired improvements in the electrical insulating and thermal stabilizing properties of the cellulosic insulation.

Second, the stabilizing compound or compounds, if more than one is used, may be present in substantially uniform distribution, that is being intimately present throughout the interstices of the fibers comprising the cellulosic insulation, to obtain maximum benefits.

Inasmuch as the stabilizing compounds of this invention are soluble in water or water-ethyl alcohol mixtures, they may be desirably incorporated in the insulation during its manufacture or fabrication. In the case of paper insulation particularly incorporation of the compounds may be made in the paper mill. Paper is generally made on either a Fourdrinier machine or a cylinder type machine. With either method, the formed web of felted cellulosic fibers is transferred from the forming screen to a felt belt for drying. During drying the web passes between the calendar rolls to impart a particular surface finish or density after which it is stored for shipment.

In practicing the invention with respect to paper insulation, the stabilizing compounds in substantially aqueous solution are added to the partially dried paper which absorbs a predetermined amount of the stabilizing compounds. The usual solution temperature is about 60 to 90 C. to produce a suitable concentrated solution. After this treatment the paper passes through a second portion of the dryer.

The following example is illustrative of the present invention.

EXAMPLE I Five squares of electrical grade kraft paper, each square bing four inches by four inches in area and five mils thick with a density of approximately 1, were dipped into a solution one at a time which solution contained about 3 parts by weight of gamma-amino propyltriethoxy silane in about 100 parts by weight of distilled water. The

time of immersion was long enough to wet each square of paper completely. After removal from the solution the squares were dried in room air to remove all traces of the water.

Each of the samples of paper was wound with enameled wire into a coil and sealed in a vessel filled with transformer oil in which strips of transformer core iron were also placed. Sufiicient current was circulated through the coil to generate temperatures of about C. The coil unit was removed after 7 days and the Mullen bursting strength test run on the aged paper. The table lists the percent strength retention of the aged samples as compared to the Mullen bursting strength of fresh or unaged kraft paper.

Other nitrogenous silane compounds embraced in the generic formula (a) given above were obtained from the trade, applied to paper as in Example I, and tested with the following results:

before Mnllens Percent Trademark test after test retention Y-2765 (Union Carbide Corp.) 54 44 81. 5 Y-2764 (Union Carbide Corp.) 7O 49 70. 0 Z6020+OZ83026 (Dow Corning) 70 58 82.9 22 (Dow Corning) 58 35 60. 3 23 (Dow Corning) 57 53 93. 0 Untreated Kraft 72 28 38. 9

The other Examples 1 to 5 of compounds and polymers can be substituted for the silane in Example I with equally good results.

There is a relationship between the mechanical strength retention and the dielectric properties retention for the cellulosic materials of the invention since an integrally strong paper has good dielectric properties. Up to 5% of a resin can be added to the cellulosic material containing the stabilizing compound of the invention and thereby further enhance the mechanical, electrical, and thermal stability of the finished product. For example, polyacrylamide resin or melamine aldehyde resin may be incorporated in the stabilized paper.

It is to be understood that the above description is illustrative and not in limitation of the invention.

What is claimed is:

l. A thermally stabilized cellulosic insulation material suitable for use in a transformer in the presence of air and liquid dielectric which material is stabilized against loss of physical strength with passage of time at elevated temperatures, the material comprising a cellulosic material having incorporated therein about 0.02% to about 10% by weight, based on the weight of the cellulose material, of a stabilizer consisting of a liquid nitrogencontaining organosilane selected from at least one of the group consisting of:

where X represents amino and cyano (CN) group, n is 1 or 0, and in has a value of from 0 to 25, and R is an alkyl or aryl radical, and R R and R are alkyl, alkoxy, aryl or aryloxy radicals.

2. The cellulosic insulation material of claim 1 in which the material is cellulosic paper and the amount of stabilizing compound is from about 0.5 to about 5% by Weight.

3. The insulation of claim 1 wherein the cellulosic material incorporates up to 5% of a resin to provide for improved mechanical, electrical, and insulating of the material.

4. The cellulosic insulation material of claim 1 in which the stabilizing compound comprises gamma-amino propyltriethoxy silane having thegeneral formula:

0 CZHE NHzCHzCHz CHz-Si-O C2115 References Cited UNITED STATES PATENTS McCulloch.

Petley 252-63.7 X Bailey 25263.7 X Ford.

Ford.

Sadler.

Rochow 252-63.7 X Ford.

Sadler.

Cooper 117--143 X WILLIAM D. MARTIN, Primary Examiner W. R. TRENOR, Assistant Examiner US. Cl. X.R. 

